Timely activation and inactivation of Cyclin dependent kinases (CDKs) regulate most cell cycle transitions. Precise coordination of each cell cycle step with the others is essential for cells to correctly transmit an intact genome to each daughter cell. Failure to do so can lead to polyploidy, genomic instability and cancer. The function of phosphatases which reverse the action of CDKs is less well understood. The highly conserved Cdc14-family phosphatases act to reverse Cdk phosphorylation events. Most of our knowledge about Cdc14 phosphatases has come from yeast. Key unanswered questions about this family of phosphatases are how they are regulated, what are their targets, and how does the phosphatase regulate these targets to promote key steps of mitosis and cytokinesis? We have been studying the fission yeast Cdc14 homolog, known as Clp1. Our work previous work showed that Clp1 activity is closely integrated with the activities of other key cell cycle regulators. In addition, we found that Clp1 is required to promote proper chromosome segregation, cytokinesis, and inactivation of Cdk1 at the end of mitosis. These studies have identified potential targets for Clp1 involved in each of these important steps. In this proposal we will exploit these results to define in molecular terms how Clp1 is regulated, and how it acts to promote chromosome segregation, cytokinesis, and Cdk1 inactivation. The involvement of Clp1 in these various steps in cell cycle progression makes Clp1 a central regulator for controlling key cell cycle events and promoting genomic stability. Our specific aims are: 1) To test the model that proper chromosome segregation requires a carefully regulated equilibrium between Cdk1 phosphorylation and Clp1 dephosphorylation of Survivin and INCENP. 2) To test a hypothesis for how the Sid2 kinase and the 14-3-3 protein, Rad24, maintain elevated Clp1 activity until cytokinesis is complete. 3) To test a hypothesis that Clp1 promotes actomyosin ring stability through effects on Cdc15. 4) To determine how Clp1 and the anaphase promoting complex promote Cdc25 inactivation in late mitosis.